1. Field of Invention
The invention relates to a Helium-Neon gas laser and an optical system used therewith.
2. Description of Related Art
Typically gas lasers are used in apparatus which rely on the specific frequency of the laser light, for example light having a known wavelength can be used for interferometric measurement. Laser light at a specific frequency is used as a reference to measure the frequency of other light for example in heterodyne frequency measurement systems. Laser spectroscopy requires a light of narrowly defined frequency also. Laser light of a specific polarisation and frequency can be used for polarisation measurement.
Helium-Neon (HeNe) lasers are well suited to these applications since they produce a convenient frequency and are readily controllable.
Laser interferometers are shown in U.S. Pat. Nos. WO98/05955 and U.S. Pat. No. 4,844,593. One subject discussed in these documents is the prevention or reduction of laser output light being reflected back toward the laser (known as “optical feedback” or “back-reflection”) The amount of back-reflection can be determined from known optical parameters of the optical elements used. However, only a small proportion of back-reflected light reaches the laser cavity i.e. approximately 1 to 5 hundredths of the back-reflected light.
Back-reflection is undesirable in the devices mentioned above and in all devices which require a specific frequency of laser light, because excessive back-reflection interacts with the laser to change the polarisation and output frequency of the laser light. Various HeNe lasers suffer from sensitivity to back-reflection.
HeNe lasers having gas mixes of varying proportions are known. U.S. Pat. No. 4,475,199 describes a ring laser having a HeNe mix consisting of dual isotopes of Ne20 and Ne22. Equal proportions of these two isotopes are mixed with the He. It has now been recognised by the inventor that this mixture when used in a linear laser gives good polarisation stability and hence frequency stability in the resonant cavity of the laser when subjected to back-reflections. Therefore a laser of this type is ideal for use in back-reflective situations encountered in the devices described above.